This invention relates to a passive electrical article, circuit articles thereof, and circuit articles comprising the passive electrical article. The passive electrical article comprises at least two self-supporting substrates having an electrically insulating or electrically conducting layer between the substrates.
A continuing trend in the electronics industry is the miniaturization of electronic circuits, and the drive toward higher and higher circuit element density. On conventional printed wiring boards today, a large fraction of the surface area is occupied by surface-mounted capacitors and other passive devices. The industry has recognized that one way to further increase circuit element density is to eliminate surface-mounted passives and embed or integrate passive structures in the circuit boards themselves. This has the added advantage of placing the capacitors much closer to the active components, thus reducing electrical lead length and lead inductance, thereby improving circuit speed and reducing signal noise. Examples of embedded or integrated capacitor articles are disclosed in U.S. Pat. Nos. 5,010,641; 5,027,253; 5,079,069; 5,155,655; 5,161,086; 5,162,977; 5,261,153; 5,469,324; 5,701,032; 5,745,334; and 5,796,587.
A basic capacitor construction consists of two electrically conductive electrodes separated by a thin layer of electrically insulating dielectric material. In present embedded capacitor technologies, the dielectric material is typically an anodized or sputter-deposited metal oxide, such as tantalum oxide, or a high dielectric constant ceramic, such as barium titanate, dispersed in a matrix of some thermally and mechanically stable polymer, such as an epoxy.
It is known that for the polymer-based capacitors to have satisfactory mechanical strength and interlayer adhesion, the metal electrodes must have rough surfaces. These rough surfaces limit the minimum thickness possible without creating short circuits (xe2x80x9cshortsxe2x80x9d) and high leakage currents across the capacitor structure, since otherwise, random protrusions on the two facing electrode surfaces could bridge the gap across the dielectric layer and make contact.
Capacitance, C, of a parallel plate capacitor is given by the equation: C=KA/4xcfx80d, where K represents the dielectric constant of the medium between the plates, A represents the area of the plates, and d represents the distance between the plates. Accordingly, capacitance per unit area (measured typically in nF/cm2) can only be increased by reducing the dielectric layer thickness (electrode spacing) of the capacitor or increasing the dielectric constant of the dielectric material between the conductive electrodes. Thus, it was believed that a higher capacitance per unit area, which is increasingly required for modern high frequency, high speed circuits, could only be achieved in polymer-based capacitors by using dielectrics with unusually high dielectric constants.
It is well known that capacitors can be formed by placing a layer of a high dielectric constant ceramic dispersed in an organic polymer between two conductive electrode sheets, e.g., barium titanate in epoxy between copper foils. Such capacitor sheets or laminates can be used as a layer in printed wiring boards and multichip modules to replace surface mounted discrete capacitors. Such capacitor sheets are currently sold; however, they have low capacitance (typically less than 1 nF/cm2) which limits their usefulness. Two well known ways of increasing the capacitance of such a laminate are to decrease the coating thickness and to increase the dielectric constant. To be useful, coating thicknesses typically need to be in range of 1 to 10 micrometers (xcexcm) with a ceramic volume loading of approximately 50%. Commercially available capacitor laminates have a 50 to 100 xcexcm thick dielectric layer.
The industry continues to seek a capacitor article possessing mechanical strength and chemical resistance sufficient to withstand circuit fabrication and handling processes; dielectric layers with high dielectric constants which are stable over wide frequency and temperature ranges; thin dielectric layers to achieve the high capacitance sometimes needed; and low direct current (DC) leakage current, low loss and high breakdown characteristics, which do not have DC electrical contact or xe2x80x9cshort circuitsxe2x80x9d across the appreciably large dielectric layer areas (e.g. several square centimeters) and which are unaffected by environmental conditions used in service or qualification testing.
The drive toward reducing circuit size and minimizing lead inductance has also spurred an interest in developing similar articles with resistive function.
The present invention is directed to a passive electrical article, such as a capacitor or resistor, which may be embedded or integrated within a circuit or which may function as an electrical circuit.
In one embodiment, the present invention relates to a passive electrical article comprising (a) a first self-supporting substrate having two opposing major surfaces, (b) a second self-supporting substrate having two opposing major surfaces, and (c) an electrically insulating or electrically conducting layer comprising a polymer and having a thickness ranging from about 0.5 to about 10 xcexcm between the first and second substrate. A major surface of the first substrate in contact with the layer and a major surface of the second substrate in contact with the layer have an average surface roughness ranging from about 10 to about 300 nm. A force required to separate the first and second substrates of the passive electrical article at a 90 degree peel angle is greater than about 3 pounds/inch (about 0.5 kN/m).
The passive electrical article may be patterned to form an electrical circuit or may further comprise an electrical contact to form an electrical circuit. In addition, the present invention relates to a printed wiring board or a flexible circuit comprising the passive electrical article as well as to an electrical device comprising a printed wiring board or a flexible circuit which comprises the passive electrical article.
The present invention also relates to a method of manufacturing a passive electrical article comprising the steps of: (1) providing a first metal substrate, having two opposing major surfaces, substantially free of debris or chemisorbed or adsorbed materials and a second metal substrate, having two opposing major surfaces, substantially free of debris or chemisorbed or adsorbed materials, (2) providing a blend comprising a resin, (3) coating the blend onto a first major surface of the first substrate so that the blend, after curing or drying, forms a layer having a thickness ranging from about 0.5 to about 10 xcexcm, (4) laminating the first major surface of the second substrate or a first major surface of the second substrate coated with the blend onto the first major surface of the first substrate, and (5) curing or drying the blend. The first and second substrates may be annealed before step (2) or as a consequence of step (5). The first major surface of the first substrate and the first major surface of the second substrate have an average surface roughness ranging from about 10 to about 300 nm. A force required to separate the first and second substrates of the passive electrical article at a 90 degree peel angle is greater than about 3 pounds/inch (about 0.5 kN/m).
The present invention is unique because the passive electrical article comprises a relatively thin electrically insulating or electrically conducting layer in combination with relatively smooth substrates and still achieves an adhesion as described herein.